I did 2 mileage runs to MUC in December, during the second I was there 2 nights (not exactly a pure MR), but did a daytrip to Salzburg. I'd highly recommend it. Do a little googling on the Bayern Pass (train ticket) on some other cheap/easy options. There are a bunch of small museums in Salzburg, churches, and an inclined train that goes up to a fortress. I've made friends jealous showing pictures of the mountains from that fortress. The train ride between Munich and Salzburg is very scenic also.

If you haven't yet been to BMW World (while in Munich), that is a great place to spend a few hours looking over some great cars.